Iran was accused today of hampering international officials engaged in inspecting its nuclear facilities.

As the country's Islamic rulers prepared to present their formal response to an international incentive offer aimed at persuading them to abandon a nuclear fuel project the west suspects is designed to produce an atomic bomb, western diplomats said inspectors from the International Atomic Energy Agency in Vienna were refused access to areas of Iran's uranium enrichment centre at Natanz.

Western diplomats and officials familiar with the IAEA inspections, however, played down the problems of access and agreed that the Iranians were entitled for the time being to refuse access to an underground hall at Natanz being built to host an industrial-scale enrichment facility.

They denied reports that the access refusal amounted to a violation of Iran's obligations under the nuclear non-proliferation treaty, which is policed by the IAEA.

The prospects, meanwhile, for the international offer to Iran producing a breakthrough deal appeared dim after Iran's supreme leader, Ayatollah Ali Khamenei, declared on Monday that the country would continue "on the nuclear path".

"The Islamic Republic of Iran has made its own decision and in the nuclear case, God willing, with patience and power, will continue its path. It will receive the sweet fruits of its efforts," he said on state television.

His announcement followed statements by government officials that Iran would reject demands to suspend uranium enrichment, which the UN security council had set as an essential condition of any agreement.

In a graphic expression of the hardening Iranian mood, a senior MP, Alaeddin Boroujerdi, head of the parliament's national security and foreign policy committee, said Iran would formally halt IAEA inspections if international pressure continued.

"If the pressure on Iran increases, the activities of the IAEA in Iran's nuclear installations will be restricted," he said. "If the Europeans act hastily and ignore Iran's national rights, agency inspections, as required by the NPT, would not have any force. In the event of sanctions, parliament will approve a bill requiring the removal of agency cameras from our nuclear installations."

The UN incentive offer, put together by the security council's five permanent members of America, Britain, France, Russia and China, plus Germany, offers Iran a range of economic sweeteners, including civilian nuclear technology.

However, a council resolution passed last month gave Iran until August 31 to suspend enrichment or face the prospect of sanctions. Uranium enrichment is a process which can be used to fuel domestic power stations or - conducted to a high enough level - produce atomic bombs.

Iranian officials have been encouraged in their defiance by high oil revenues and the known reluctance of Russia and China, both of whom have strong economic ties with Tehran, to countenance tough embargoes. The benchmark Brent crude oil price rose $0.75 yesterday to $73.05 a barrel on the strength of the comments from Mr Khamenei.